In subterranean wells for recovering oil and natural gas deposits, the use of drilling fluids and cement slurries has been known for some time. When the borehole has reached a certain depth, a casing is introduced into the hole. For this purpose, the casing must be secured, i.e. a cement slurry is pumped into the space between the formation and the casing and sets to form a solid rock. The hardened cement must be impermeable to gases and liquids in order that no gas and/or oil can flow out of the carrier rock into other sections or as far as the surface. The cement slurry to be pumped is subject to high demands. It should be readily pumpable, i.e. of the lowest possible viscosity, and nevertheless should not separate out. The release of water by the cement slurry to the porous rock should be low during the pumping operation, in order that thick filter cakes do not form at the borehole wall; thick filter cakes would increase the pumping pressure so much, owing to the annular space constriction, that the porous rock would disintegrate. In addition, the cement slurry would not set properly and would become permeable to gas and oil if the release of water were excessive. On the other hand, the cement jacket forming must reach a certain strength as rapidly as possible in the annular space, and shrinkage must not occur during setting, as this would result in flow channels for gas, oil and water. Optimum setting of the properties of the cement slurry is only possible through additives. The most important additives are retardants, accelerators, dispersants and water-loss reducers.
Effective water-loss reducers used in practice in cement and gypsum slurries are a wide variety of polymers, copolymers and combinations thereof. The first effective products, which are still used today, were cellulose ethers based on hydroxyethylcellulose and carboxymethylhydroxyethylcellulose. Owing to thermal instability, these lose their efficiency at borehole temperatures above 100.degree. C. (212.degree. F.). As a consequence, many different fully synthetic heat-stabilized polymers have been developed and are still in use today at the various temperatures and salinities of the cement slurry.
Polymers as additives for reducing water loss in cement slurries are well known from the literature. Many water-loss reducers have greatly restricted activity at high temperatures.
U.S. Pat. No. 2,614,998 describes the use of partially hydrolyzed polyacrylamide (poly(acrylamide-co-acrylic acid) as water-loss-reducing polymers. However, these polymers can result in considerable retardation of the cement setting time and have only low effectiveness at high temperatures.
U.S. Pat. No. 2,865,876, U.S. Pat. No. 2,905,565 and U.S. Pat. No. 3,052,628 describe the use of sulfonated polymers as additives. The polymers and copolymers described therein differ significantly in their composition from the copolymers according to the present invention and have achieved absolutely no industrial importance.
U.S. Pat. No. 5,472,051 describes copolymers of AMPS and acrylic acid having molecular weights of less than 5000 g/mol.
U.S. Pat. No. 4,015,991 describes a polymer of AMPS and acrylamide, where at least 20% of the acrylamide units must subsequently be hydrolyzed to acrylic acid or a salt of acrylic acid. The claimed polymer consists of AMPS, acrylamide and acrylic acid or a salt of acrylic acid. U.S. Pat. No. 4,015,991 thus claims the following copolymer: ##STR3##
where x is 10-90 mole percent PA0 y is 10-90 mole percent PA0 z, depending on y, is 2-90 mole percent, PA0 where, if z is 0, inadequate water-loss-reducing properties were found in the test carried out. However, the disadvantage of this polymer is an undesired effect on the cement properties (reduction in the strength of the set cement) and retardation of cement setting. A further problem is the restricted temperature range for use as water-loss-reducing polymer. At 350.degree. F., it is demonstrably inactive. U.S. Pat. No. 4,015,991 showed that suitable copolymers cannot be prepared by aqueous polymerization of AMPS and acrylamide without a hydrolysis step.
EP 0 116 671 (=DE 3302168) discloses the introduction of 5-60% by weight of a further, new comonomer, namely a vinylamide (for example N-vinylmethylacetamide). This allows the high-temperature application range to be significantly broadened, but these polymers exhibit worse applicational properties at temperatures below about 38.degree. C. (100.degree. F.).
U.S. Pat. No. 5,025,040 describes copolymers of AMPS, acrylamide and at least 20% of N-vinylimidazole as new vinylamide components.
U.S. Pat. No. 4,931,489 discloses copolymers of substituted acrylamides and N-vinylimidazoles without the use of AMPS as comonomer.
EP-A-0 217 608, U.S. Pat. No. 4,555,269 and EP-A-0 157 055 describe a copolymer of AMPS and dimethylacrylamide in a molar ratio of from 1:4 to 4:1 as fluid-loss additive for cement slurries containing salts (approximately 10% by weight) and the use of AMPS and acrylic acid in a molar ratio of from 1:4 to 4:1 for the same purpose.
According to U.S. Pat. No. 5,294,651, the disadvantages of the process of U.S. Pat. No. 4,015,991 are overcome by an improvement analogous to EP-A-0 116 671 by using a further comonomer, in this case a cyclic vinylamide (for example N-vinylpyrrolidone). The proposed solution does not differ significantly from the method indicated in EP-A-0 116671.
A technically satisfactory solution for a temperature range of from 4 to 205.degree. C. (39 to 400.degree. F.) based on the monomers disclosed in US-4,015,991 or without partial hydrolysis of the acrylamide has hitherto not been described.
The multiplicity of polymers developed in order to reduce the release of water makes it clear that it is always difficult to formulate a technically optimum cement slurry. A significant effect on its suitability is exerted by the temperature of the borehole section prepared for cementing. The polymers designed for different temperatures represent considerable logistical problems, since a certain stock of a number of water-loss-reducing polymers must always be maintained at locations scattered throughout the world. The object was therefore to develop polymers which can cover the entire temperature range (4.degree. C.-205.degree. C.) and are suitable for a wide range of cement slurries, i.e. various cement qualities, mixing water salinities and cement slurry densities.
Surprisingly, it has been found that the technical properties required can be achieved by an AMPS-acrylamide copolymer if the polymerization process is modified and the subsequent hydrolysis is omitted, giving a novel polymer which has not been described before. In addition, this polymer does not cause setting retardation at below 40.degree. C.